Camila Belle was told to hit Tommy Flanagan during the scene in the atrium. She missed him and punched the bridge instead, cutting her knuckle and scarring her hand. She also really did slam her head into a glass window during the fight on the stairs.
When Tiffany (Katie Cassidy) first appears in the movie, she's stirring a jar of red paint while standing next to Scarlet - behind them are several jars of red tinted paints. The name for a woman who is promiscuous or loose is is often referred to as "a scarlet woman", a subtle reference to what Tiffany has done to betray Jill.
While in the pond under the walking bridge, Jill's bangs go from pushed to the side to down in her eyes, to pushed to the side with no obvious reason for the movement.
After Jill sees her friends body on bathroom floor, she slides down the shower curtain sliding it to a folded state. When she gets back up to run out of the room, the shower curtain is in the exact position it was in before she pushed it aside.